1. Field of the Invention
The present invention relates to a solar energy generation system, and more particularly, to a solar energy generation system tracking a maximum power point and a method thereof.
2. Description of the Related Art
The output voltage of a photovoltaic module used for solar energy generation varies depending on various environment factors. Of the environment factors, the amount of solar radiation and temperature may be regarded as the biggest factors. Referring to FIGS. 1 and 2, it can be understood in the voltage-power characteristic curve of a photovoltaic module that increasing the voltage increases the power together by a maximum power point, but increasing the voltage over the maximum power point conversely decreases power.
The reason why the photovoltaic module shows the pattern of a voltage-power characteristic curve as described above is that current and voltage in the photovoltaic module has a non-linear relationship. Due to the non-linear relationship of current and voltage in the photovoltaic module, and uncertain environment factors, including the amount of solar radiation and temperature of the photovoltaic module, it is virtually impossible to fix the maximum power point of the photovoltaic module in advance.
For this reason, various algorithms have been developed to control a solar energy generation system to operate on the track of the maximum power point according to the operation of the photovoltaic module. Algorithms which are the most widely used in universal inverters include a Perturb and Observe (P&O) scheme, an incremental conductance scheme, and a constant voltage scheme.
Of these algorithms, the P&O scheme, which is the most widely used, is to perturb the voltage of an inverter and then to determine a maximum power point through a change in the power component depending on the perturbation.
Referring to FIG. 3 which illustrates a voltage-power characteristic curve, in Cases 1 and 2, the power value decreases through adjustment of the voltage value and thus becomes more distant from the maximum power point. Specifically, Case 1 shows that a decrease in the voltage results in a decrease in the power value, and Case 2 shows that an increase in the voltage results in a decrease in the power value.
That is to say, in Cases 1 and 2, since the variations in the voltage result in decreases in the power value, it is possible for the power value to approach the maximum power point by adjusting the output voltage of the inverter in a direction opposite to that in which each corresponding variation in the voltage is made.
In contrast, in Cases 3 and 4, the power value increases through adjustment of the voltage value and thus gets near to the maximum power point. Specifically, Case 3 shows that a decrease in the voltage results in an increase in the power value, and Case 4 shows that an increase in the voltage results in an increases in the power value.
That is to say, in Cases 3 and 4, since the variations in the voltage increase the power value together, it is possible for the power value to further approach the maximum power point by adjusting the output voltage of the inverter in the same direction as that in which each corresponding variation in the voltage is made.
TABLE 1CaseΔPΔVAct.1<0<0+2<0>0−3>0<0−4>0>0+
Here, “Act” represents an action which the inverter should take to track the maximum power point, wherein “+” represents increasing the output voltage of the photovoltaic module, and “−” represents decreasing the output voltage of the photovoltaic module.
In the conventional solar energy generation system, an inverter is designed to have the minimum point in the maximum power point (MPP) range in consideration of the allowable maximum value of the grid voltage which is decided in each country. This is because of the principal reason that the minimum value of the output voltage of a photovoltaic module must secure the allowable maximum value of a grid voltage in order to operate a photovoltaic inverter connected with a grid. However, due to a reason on grid operation, some grid voltage may be maintained at a higher level than that of a rated voltage, and in severe cases, may approach the neighborhood of an allowable maximum value. In such a situation, the grid voltage may temporarily exceed the grid voltage by a generation action of a distributed generation system, such as a solar energy generation system.
When the grid voltage exceeds the allowable maximum value while the conventional photovoltaic inverter is operating at the minimum point within the maximum power point range, the operation of the inverter is stopped due to a shortfall in DC voltage. As the inverter is stopped, supplying generation energy to the grid is stopped, so that the grid voltage can return to a value within the allowable range. Since the grid voltage returns to a normal state, the inverter again starts the operation thereof, but the inverter is again stopped due to the problem as described above. Such an abnormal operation in a distributed generation system may cause a problem in the safety of the entire grid including the system.